Mesoporous TiO2 nanoparticles terminated with carbonate-like groups: Amorphous/crystalline structure and visible-light photocatalytic activity
Graphical abstract
High visible-light photocatalytic activity is obtained mesoporous TiO2 nanoparticles modified with carbonate-like groups, facilitated by the production of hydroxyl radicals and the separation of electrons and holes.
Introduction
Utilization of solar energy and semiconductor photocatalysis technique are promising in solving the energy crisis and environmental pollution [1], [2]. It is known that ultraviolet (UV) light only accounts for less than 5% and visible light about 43% in sunlight spectra. Therefore, photocatalysts with high visible-light activity stimulate incessant research interests. TiO2, one of the important and widely studied inorganic semiconductor materials in photocatalysis and photovoltaics, has a large band gap (about 3.2 eV) and only responds to the UV light [3]. A number of attempts have involved in enhancing its photocatalytic performance in the visible-light region. Introducing alien atom (metal or non-metal element) into the TiO2 lattice is a significant approach to harvest visible light [1], [4], [5], [6]. Recently, carbonate-doping has demonstrated a visible-light photocatalytic activity on hydrogen production [7] and the degradation of Rhodamine B (RhB) [8]. Apart from this, dopant-free approaches have been paid more attention to improve the visible light response. For example, loading noble metal nanoparticles on TiO2 to produce plasmon-induced photocatalytic activity [9], [10], formation of carbon-based nanocomposite [11], [12], [13], and tuning the surface defects e.g. oxygen vacancies [14], etc. Chen et al. innovatively engineered a disorder layer on anatase TiO2 nanocrystals by high-pressure hydrogenation to improve the visible-light and infrared absorption [15]. Self-doping Ti3+ could also stable oxygen vacancy and enhance the visible light photocatalytic activity [16].
In addition, mesoporous nanomaterials with high surface area and well-defined porous structure have attracted great interests in photocatalysis, because the photocatalytic efficiency of catalysts highly depends on the surface area, pore structure, crystallinity and morphologies [17], [18]. Traditionally, synthesis of mesoporous TiO2 employs templates (organic surfactant and mesoporous silicas) [19], [20] and needs post-treatment, calcination at high temperature or alkaline etching to increase the crystallinity and remove the templates. These processes can result in the aggregation of particles and the collapse of mesoporous structure, which is harmful to the photocatalytic activity of materials. Thus, facile approaches are highly needed to prepare the mesoporous TiO2 with high photocatalytic activity in visible light.
Here, we report a facile synthesis of mesoporous anatase TiO2 nanoparticles terminated with carbonate-like groups for visible-light photocatalysis. The presence of carbonate-like groups leads to the amorphous layer covering on anatase TiO2 lattice and contributes to visible-light absorption. The photocatalytic activity was evaluated by the degradation of RhB and colorless salicylic acid (SA) under visible light (wavelength > 400 nm). The charge transfer and active radicals were also investigated in details.
Section snippets
Synthesis of mesoporous TiO2 nanoparticles
The target material was synthesized by modified non-hydrolytic route we previously reported [21]. Typically, TiCl4 (≥98.0%, C.P., Sinopharm Chemical Reagent Co., Ltd) and anhydrous ethanol (≥99.7%, Sinopharm Chemical Reagent Co., Ltd) were used without further purification. 0.5 mL TiCl4 was slowly added to 40 mL of ethanol at ambient conditions by vigorously stirring, producing a completely transparent yellow solution. The solution was transferred into a Teflon-lined, stainless autoclave at 120 °C
Crystalline phase and structure
The crystallographic structures of as-prepared samples are shown in Fig. 1a. The XRD patterns of all samples were indexed to the tetragonal anatase TiO2 with a space group of I41/amd (JCPDS No. 21-1272). The intensities of diffractive peaks for TiO2-250 and TiO2-400 were higher than those of TiO2-120, indicating the post-calcination enhancing the crystallinity. According to the Scherrer equation τ = (Kλ)/(β cos θ), where τ is the mean size of the ordered (crystalline) domains, K is the shape factor
Conclusions
In summary, mesoporous anatase TiO2 nanoparticles terminated with carbonate-like groups were synthesized by a facile non-hydrolytic method following a heat-treatment at low temperature. The formation of amorphous/crystalline structure enhanced the absorption of mesoporous TiO2 nanoparticles in visible-light region and resulted in the slight narrowing in band gap (about 0.14 eV). High photocatalytic activity was obtained on the degradation of RhB (90.2% for 60 min) and SA (64.1% for 180 min) under
Acknowledgements
L. Tian gratefully acknowledges the support from National Natural Science Foundation of China (No. 51302072), Natural Science Foundation of Hubei Provincial Department of Education (No. Q20131010), Natural Science Fund for Creative Research Groups of Hubei Province of China (No. 2014CFA015). X. Chen thanks the support from College of Arts and Sciences, University of Missouri - Kansas City and the University of Missouri Research Board.
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